Optical signals carry information with on/off keying. Due to the physics of the turn on transition wavelength chrip is a problem for direct modulated lasers. One way of minimizing chrip is to make the OFF state not completely off. The ratio of ON to not completely OFF is known as the extinction ratio. The extinction ratio is a useful measurement for an optical signal transmitter. One technique for measuring extinction ratio utilizes an expensive analyzer having a fast response time for performing optical to electrical conversion.
Another technique for measuring extinction ratio is set out in U.S. Pat. No. 5,502,298 to Geller. Geller is directed to a circuit for controlling an extinction ratio of a laser whose temperature can change over time. The circuit includes first and second feedback loops which monitor a LOW output power of the laser during a first frame training pulse and a HIGH laser output power during a second frame training pulse. The LOW power output is compared to a present dynamic LOW power reference, and a LOW bias current applied to the laser is incrementally increased or decreased so as to keep this LOW power output toggling about this LOW reference. Similarly, the HIGH power output is compared to a HIGH power reference, and a modulation current for the laser is incrementally increased or decreased to keep the HIGH laser output power toggling about this HIGH reference. Preferably, the training pulse is sent once per frame thus enabling both the LOW and HIGH laser output powers to be kept constant regardless of whatever dynamic variables may change over time thus keeping the laser extinction ratio constant.